Fix config binder source gen for a sole read-only collection ctor param

src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Specs/BindingHelperInfo.cs

@@ -169,7 +169,17 @@ bool TryRegisterCore()
                                 // an error for config properties that don't map to object properties.
                                 _namespaces.Add("System.Collections.Generic");
 
-                                if (_typeIndex.HasBindableMembers(objectSpec))
+                                bool hasBindableMembers = _typeIndex.HasBindableMembers(objectSpec);
+
+                                // A type with a parameterized constructor gets its constructor parameters bound
+                                // in the Initialize method regardless of whether it also has other bindable
+                                // members; that binding capability must be registered even when
+                                // HasBindableMembers is false (e.g. the only member is a ctor parameter backed
+                                // by a non-bindable read-only collection type), otherwise the emitter can end up
+                                // calling an Initialize method that was never generated.
+                                bool needsInitializeMethod = objectSpec is { InstantiationStrategy: ObjectInstantiationStrategy.ParameterizedConstructor, InitExceptionMessage: null };
+
+                                if (hasBindableMembers || needsInitializeMethod)
                                 {
                                     foreach (PropertySpec property in objectSpec.Properties!)
                                     {
@@ -189,10 +199,13 @@ bool TryRegisterCore()
                                         }
                                     }
 
-                                    bool registeredForBindCore = TryRegisterTypeForBindCoreGen(objectSpec);
-                                    Debug.Assert(registeredForBindCore);
+                                    if (hasBindableMembers)
+                                    {
+                                        bool registeredForBindCore = TryRegisterTypeForBindCoreGen(objectSpec);
+                                        Debug.Assert(registeredForBindCore);
+                                    }
 
-                                    if (objectSpec is { InstantiationStrategy: ObjectInstantiationStrategy.ParameterizedConstructor, InitExceptionMessage: null })
+                                    if (needsInitializeMethod)
                                     {
                                         RegisterTypeForMethodGen(MethodsToGen_CoreBindingHelper.Initialize, objectSpec);
                                     }

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/SourceGenerationTests/GeneratorTests.Helpers.cs

@@ -9,6 +9,7 @@
 using System.IO;
 using System.Linq;
 using System.Reflection;
+using System.Runtime.Loader;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis;
@@ -230,5 +231,21 @@ private static byte[] CreateAssemblyImage(Compilation compilation)
             }
             return stream.ToArray();
         }
+
+        /// <summary>
+        /// Compiles the source-generated output to an in-memory assembly, invokes its Program.Main(),
+        /// and returns the value of the public static field named <paramref name="resultFieldName"/>.
+        /// Used to assert on real bound values rather than just that the generated code compiles,
+        /// since a generator can emit code that builds cleanly but binds the wrong data.
+        /// </summary>
+        private static object? LoadAndInvokeMain(Compilation compilation, string resultFieldName)
+        {
+            byte[] image = CreateAssemblyImage(compilation);
+            Assembly assembly = AssemblyLoadContext.Default.LoadFromStream(new MemoryStream(image));
+
+            Type programType = assembly.GetType("Program")!;
+            programType.GetMethod("Main", BindingFlags.Public | BindingFlags.Static)!.Invoke(null, null);
+            return programType.GetField(resultFieldName, BindingFlags.Public | BindingFlags.Static)!.GetValue(null);
+        }
     }
 }

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/SourceGenerationTests/GeneratorTests.cs

@@ -542,6 +542,97 @@ public record Options(string Name, {{collectionType}}<string> Values);
             AssertCanCreateAssemblyImage(result.OutputCompilation);
         }
 
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNetCore))]
+        [InlineData("IReadOnlyList")]
+        [InlineData("IReadOnlyCollection")]
+        [InlineData("IReadOnlySet")]
+        [InlineData("IEnumerable")]
+        public async Task SoleReadOnlyCollectionConstructorParameterIsBindable(string collectionType)
+        {
+            // Regression test: a type whose only member is a non-bindable, read-only collection
+            // constructor parameter (no other bindable property) used to make the generator emit a
+            // call to an Initialize method that was never generated, producing CS0103 at compile time.
+            string source = $$"""
+                using Microsoft.Extensions.Configuration;
+                using System.Collections.Generic;
+
+                public class Program
+                {
+                    public static object? Result;
+
+                    public static void Main()
+                    {
+                        ConfigurationBuilder configurationBuilder = new();
+                        configurationBuilder.AddInMemoryCollection(new Dictionary<string, string?>
+                        {
+                            ["Values:0"] = "a",
+                            ["Values:1"] = "b",
+                        });
+                        IConfiguration config = configurationBuilder.Build();
+                        Options options = config.Get<Options>();
+                        Result = options.Values;
+                    }
+                }
+
+                public record Options({{collectionType}}<string> Values);
+                """;
+
+            ConfigBindingGenRunResult result = await RunGeneratorAndUpdateCompilation(source, assemblyReferences: GetAssemblyRefsWithAdditional(typeof(ConfigurationBuilder), typeof(List<>)));
+            Assert.NotNull(result.GeneratedSource);
+            Assert.Empty(result.Diagnostics);
+
+            // Compiling only proves the Initialize method the fix registers is emitted; loading and
+            // running the assembly proves it also binds the right values, not just compilable code.
+            var boundValues = (IEnumerable<string>)LoadAndInvokeMain(result.OutputCompilation, "Result")!;
+            Assert.Equal(new[] { "a", "b" }, boundValues);
+        }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNetCore))]
+        public async Task SoleReadOnlyCollectionConstructorParameterOfComplexElementIsBindable()
+        {
+            // Same regression as SoleReadOnlyCollectionConstructorParameterIsBindable, but the element
+            // type is itself a bindable object rather than a string. ComplexReadOnlyListConstructorParameterIsBindable
+            // below covers the complex-element case, but always pairs it with a second, ordinarily-bindable
+            // property, so it never exercises the fixed code path (the type has bindable members either way).
+            string source = """
+                using Microsoft.Extensions.Configuration;
+                using System.Collections.Generic;
+                using System.Linq;
+
+                public class Program
+                {
+                    public static object? Result;
+
+                    public static void Main()
+                    {
+                        ConfigurationBuilder configurationBuilder = new();
+                        configurationBuilder.AddInMemoryCollection(new Dictionary<string, string?>
+                        {
+                            ["Values:0:Value"] = "a",
+                            ["Values:1:Value"] = "b",
+                        });
+                        IConfiguration config = configurationBuilder.Build();
+                        Options options = config.Get<Options>();
+                        Result = options.Values.Select(v => v.Value).ToArray();
+                    }
+                }
+
+                public class Child
+                {
+                    public string Value { get; set; }
+                }
+
+                public record Options(IReadOnlyList<Child> Values);
+                """;
+
+            ConfigBindingGenRunResult result = await RunGeneratorAndUpdateCompilation(source, assemblyReferences: GetAssemblyRefsWithAdditional(typeof(ConfigurationBuilder), typeof(List<>)));
+            Assert.NotNull(result.GeneratedSource);
+            Assert.Empty(result.Diagnostics);
+
+            var boundValues = (string[])LoadAndInvokeMain(result.OutputCompilation, "Result")!;
+            Assert.Equal(new[] { "a", "b" }, boundValues);
+        }
+
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNetCore))]
         public async Task ComplexReadOnlyListConstructorParameterIsBindable()
         {